The present invention relates to feedback amplifiers and, in particular, to a feedback amplifier that employs compensation circuitry which provides an amplifier with relatively wide bandwidth and relatively large gain and phase margins.
There are two common measures of stability of a feedback amplifier, which measures include gain margin and phase margin. Gain margin is defined to be the magnitude in decibels of the loop gain of the amplifier for the frequency at which the phase of the loop gain equals -180.degree.. The magnitude of the loop gain at this frequency is usually required to be less than 0 dB for stability. Phase margin is defined to be the sum of 180.degree. and the phase of the loop gain for the frequency at which the magnitude of the loop gain is unity. The phase margin at this frequency is usually required to be greater than 0.degree. for stability.
Compensation is the method by which one ensures that a feedback amplifier is stable. Dominant pole compensation is the most common method of compensation and entails the reduction of open loop amplifier bandwidth by deliberately introducing a dominant pole at a predetermined frequency into the amplifier transfer function to force the magnitude of the loop gain to unity before the phase shift increases (i.e., becomes more negative) to -180.degree. beyond its low frequency value. The frequency at which the magnitude of the loop gain is unity is denoted as "f.sub.unity. " One technique for implementing dominant pole compensation is Miller compensation, which involves a direct sacrifice of amplifier open loop bandwidth for stability.
Miller compensation of a common-emitter bipolar transistor amplifier entails the positioning of a feedback capacitor of value C.sub.M between the collector and base terminals of the transistor. The capacitor C.sub.M introduces a dominant pole at a sufficiently low frequency into the amplifier transfer function to provide a stable feedback amplifier with adequate gain and phase margins. The Miller compensation capacitor has been found in some cases to reduce the phase margin and/or gain margin of an amplifier at relatively high frequencies. This can cause, inter alia, "preshoot" in the step response of the amplifier.
Equivalently, Miller compensation can create a right half plane zero in the amplifier transfer function. The right half plane zero appears approximately at a frequency f.sub.z =g.sub.m /2.pi.C.sub.M, where g.sub.m is the transconductance of the transistor. The existence of a right half plane zero is undesirable in the gain function of an amplifier because a right half plane zero simultaneously increases the magnitude and phase shift (i.e., the phase shift becomes more negative) of the loop gain with increasing frequency. The presence of a right half plane zero is undesirable because it makes the task of compensation difficult or impossible.
One way of minimizing the adverse effects of a right half plane zero is to select a transistor topology, device type, and operating point combination that produces a large transconductance. This solution is, of course, not available for a transistor amplifier whose transconductance is inadequate to position the right half plane zero at a frequency that is much larger than f.sub.unity (e.g., typically at least five times f.sub.unity).